Earlier this evening, Twitter filed its initial public company registration form, known as the S-1, with the Securities and Exchange Commission.

In so doing, Twitter joins a small but growing club of social media stars that have gone public in recent years, including Facebook and LinkedIn.

Twitter's S-1 is the first step in filing for an initial public offering, and it also represents the first time the private company has parted the iron curtain on details about its revenue and (lack of) income.

Key Numbers Revealed

In the S-1, Twitter reported that as of June 30, it had 218 million active users.

For the full year 2012, Twitter reported $317 million in revenue, nearly three times its revenue in 2011. For the six months ended June 30 2013, Twitter reported it had $253 million in revenue, about double its revenue compared to the same six months a year earlier.

Twitter said that it earned nearly 90 percent of its 2012 revenue, and revenue for the six months reported of 2013, from advertising.

As of June 30, 2013, Twitter had $418 million in debt.

Twitter Is Not Profitable

In 2012, the company reported a net loss of $80 million, a decrease of nearly 40 percent from the prior year. For the six months ended June 30, Twitter reported a loss of $70 million, an increase of 41 percent.

That compares to Facebook's $1 billion in annual net income when it went public in May 2012.

Twitter's S-1 did not include an estimate for share price when it does list on an exchange.

Twitter's underwriting investment banks include Goldman, Sachs & Co., Morgan Stanley, J.P. Morgan, and BofA Merrill Lynch. According to SEC rules, the company's road show could begin as early as three weeks from now.

Registration Documents Confidential, at First

Twitter had previously announced its intention to file for an IPO in mid-September with this less-than-140-character Tweet that said: "We've confidentially submitted an S-1 to the SEC for a planned IPO. This Tweet does not constitute an offer of any securities for sale."

As an emerging growth company, with revenue or non-convertible debt less than $1 billion, Twitter qualified under a provision in the JOBS Act to initially file its public company registration documents with the SEC in secret.